1. Field of the Invention
This invention relates generally to tilt sensors, and more particularly, to a thermocouple tilt sensor.
2. Description of the Related Art
Tilt sensors are generally known in the art. Specifically, Crossan, Jr. (U.S. Pat. No. 5,630,280) and Seipp, Jr. et al. (U.S. Pat. No. 5,852,878) disclose two common forms of electrolytic tilt sensors. Electrolytic tilt sensors are devices that change their electrical properties when tilted as a result of the interaction between an electrolyte and a plurality of electrodes contained therein. For example, known electrolytic tilt sensors provide an output voltage proportional to the tilt angle and a phase indicative of tilt direction when the sensor is configured as part of an appropriate electrical circuit. In addition, the tilt sensors can be configured to measure acceleration. In a tilt sensor configuration, the output voltage changes based on the change of impedance between the electrodes. The impedance between each electrode changes as the tilt angle changes and either more or less electrolytic fluid surrounds the electrodes.
Referring now to FIG. 1, there is shown a three-dimensional representation of a dual axis electrolytic tilt sensor 100 according to the prior art. Tilt sensor 100 is comprised of a cylindrical housing 120 that is partially filled with an electrolytic solution 140. Within cylindrical housing 120 are disposed a common electrode 150, a pair of first axis electrodes 160 and 170 and a pair of second axis electrodes 180 and 190, wherein common electrode 150, first axis electrodes 160, 170 and second axis electrodes 180, 190 are partially immersed in electrolytic solution 140.
When sensor 100 is tilted, the surface of electrolytic solution 140 remains in a horizontal level plane with respect to gravity, and electrolytic solution 140 shifts with respect to the electrodes thereby covering the electrodes with more or less electrolytic solution. The increase or decrease of immersion in the electrolytic solution produces a corresponding change in impedance between the electrodes. This change in impedance is measured using an appropriate electrical circuit and is used to determine the change in tilt.
Tilt sensing devices, originally conceived for weapons delivery and aircraft navigation, have found a wide variety of uses. This is primarily because the tilt sensor""s voltage signal output may provide an input to a preprogrammed guidance or other system, or provide an indication of the tilt angle via an electrical signal at a location remote from the sensor.
It is also generally known that existing tilt sensors, such as the electrolytic tilt sensor, may consist of a tubular or channeled glass envelope partially filled with an electrolytic fluid having conducting metal electrodes (working and common electrodes) formed therein. The envelope configuration, construction, type of electrolyte, electrode arrangement and number of electrodes may be varied to provide the desired operating characteristics. However, it is also known that many of such existing tilt sensors produce less than accurate results and/or suffer from stability problems caused by, for example, reactions between the electrolytic fluid and the electrodes.
U.S. Pat. No. 5,581,034 to Dao et al. (Dao ""034) and U.S. Pat. No. 3,241,374 to Menkis (Menkis ""374) attempt to solve some of the problems associated with the use of electrolytic fluid by taking advantage of the convective properties of a heated gas.
Referring now to FIG. 2, there is shown a schematic diagram of a tilt sensor 200 according to Dao ""034 which utilizes the properties associated with convective gas currents. Dao ""034 discloses a convective accelerometer and inclinometer having two temperature sensing elements mounted within a sealed enclosure containing a gas. The application of heat to the gas within the enclosure by a heating element causes the gas to flow in a predetermined pattern in free convection. The less dense heated air rises and passes over sensing elements in the form of wires positioned throughout the enclosure. The resistance of a sensing element wire changes in proportion to the change in temperature, i.e. how much heated air passes over it. How much heated air passes over a sensing element is a function of tilt; thus, the resistive temperature coefficient change allows the tilt angle measurements to be performed with a corresponding electrical circuit. Convective accelerometer 200 is comprised of a sealed container 205, a first temperature sensing element 210, a second temperature sensing element 220, and a heating element 230. Heating element 230 heats a gas enclosed within sealed container 205 and the resistive temperature coefficient of the first and second temperature sensing elements 210 and 220 change in proportion to the amount of heat transferred from the heated gas. As noted, since heated air rises, the temperature sensing elements will receive more or less heat depending on whether they are rotated to a position that is partially above or partially below heating element 230. The change in the resistive temperature coefficient of the first and second temperature sensing elements 210 and 220 is measured and used to determine a corresponding tilt angle.
Referring now to FIG. 3, there is shown a schematic diagram of a device for sensing acceleration 300 according to Menkis ""374. Menkis ""374 discloses a device for sensing acceleration which is comprised of a heater located between at least two thermistors within a cylinder. When the cylinder is displaced, more or less heated gas passes over the thermistors located at the ends of the cylinder. The corresponding change in resistance of the thermistors is sensed by an appropriate electrical circuit and an output voltage is correlated to a tilt angle or acceleration.
The Menkis ""374 device 300 is comprised of a sealed enclosure 310 having an elongated tubular member 320 disposed therein. A heater 330 is located midway within elongated tubular member 320. Elongated tubular member 320 has two openings located at either end. Within each opening is disposed a bead thermistor 340, 340a for sensing heat. When device 300 moves in the direction of travel 350, heat flows in the tube as a function of acceleration of the tube. More heat will be directed to bead thermistor 340a and less heat will be directed to bead thermistor 340. The resistance of bead thermistors 340, 340a change in proportion to the amount of heat received. Thus, the amount of acceleration can be determined based upon the amount of heat received at each bead thermistor 340, 340a as measured by the corresponding change in resistance. An appropriate electrical circuit connected to bead thermistors 340, 340a measure the change in resistance and determine the amount of acceleration. Unfortunately, bead thermistors 340, 340a also have a resistive temperature coefficient property that may vary widely and are affected by external temperature sources. The resistive temperature coefficient is a factor in producing inaccurate results. Therefore, there is a need in the art for a tilt sensor device which minimizes the reliance on temperature coefficient properties.
While some of the problems associated with electrolytic tilt sensors may be solved using a gas-based tilt sensor, such as Menkis ""374 and Dao ""034, the reliance on the resistive temperature coefficient of sensor wires in determining a tilt angle brings with it its own associated drawbacks. For example, sensor wires are more sensitive to changes in outside temperatures not associated with a change in angle or inclination. In addition, the sensor elements are constructed of very thin wire and frequently break. Also, the need for precisely balanced placement of the sensor wires, the need to measure angular movements of the housing, the need for reliable performance in various operating environments, and other factors provide additional problems.
Thermocouple devices do not rely on resistive temperature coefficient properties. A thermocouple generates an electromotive force (xe2x80x9cemfxe2x80x9d) in response to a temperature difference between the two ends of a conductor. Thermocouples generate the emf because of several thermoelectric principles. One principle, known as the Peltier effect, consists of the generation or absorption (depending on the direction of the current (i)) of heat, at a rate Q, at a junction between two different conductors when a current flows through them according to the equation: Q=II i, where II is the Peltier coefficient. A second thermoelectric principle applicable to thermoelectric devices states that a thermoelectric voltage dE in an open circuit consisting of two different conductors is equal to the temperature difference dT between their ends multiplied by a thermal emf coefficient (a1-2) between the given conductors according to the equation: dE=a1-2 dT. It has also been shown that an emf can be produced in a thermocouple that is the result of a temperature gradient between the ends of one conductor and not as the result of a temperature difference between the ends of the conductors, i.e. Hot and Cold Junctions. That is, if the ends of a conductor are at the same temperature and a warmer or colder region is located between them, a potential difference will appear between the ends of the thermocouple when the wanner or colder region is closer to one of the ends of the conductor. (See, Semiconductor and Thermoelements Thermoelectric Cooling, A.F. loffe, (copyright)1957 by Infosearch, Ltd., Pages 4-13).
Still another principle affecting the output voltage of a thermocouple junction is discussed in the case where the two sensor wires are comprised of dissimilar semiconductor materials. It has been found that thermocouples made of both regular metals and semiconductors display a polarity with respect to the thermoelectric voltage generated as a result of a temperature difference between the ends of the conductors. When an external load resistance is connected between the ends of the conductors that form the Cold Junction, the Cold Junction end of the first conductor has a negative charge while the Cold Junction end of the second conductor has a positive charge. Thus, the Hot Junction end of the first conductor may be referred to as the positive conductor while the Hot Junction end of the second conductor may be referred to as the negative conductor. (See, Thermocouple Amplifiers for Signal Conditioning, (copyright)1961 by Axiomatic Technologies Corp., Pages 222-224).
As a result, it would be highly advantageous to provide an economical tilt sensing device which substantially improves operational accuracy and stability as compared to prior art devices using the principles associated with a thermocouple.
It is an object of the present invention to provide a thermocouple tilt sensing device that makes use of a controlled thermal convection gradient within a sealed housing.
It is an additional object of the present invention to provide a thermocouple tilt sensing device that uses a thermocouple to detect angular changes.
It is another object of the present invention to provide a thermocouple tilt sensing device that is superior to existing tilt sensors that rely on electrical resistance temperature coefficient properties to determine tilt angle.
It is another object of the present invention to provide a thermocouple tilt sensing device which operates using either alternating current (AC) or direct current (DC).
It is a further object of the present invention to provide a thermocouple tilt sensing device that overcomes the disadvantages associated with electrolytic tilt sensors, namely, in the use of electrolytic fluids.
It is yet another object of the present invention to provide a thermocouple tilt sensing device that is reliable, accurate, durable, and inexpensive to manufacture.
To achieve the above objects of the present invention there is provided a thermocouple tilt sensing device having at least two sensor electrodes and at least two heater wire electrodes enclosed in a sealed housing. At least two sensor wires constructed of dissimilar metals are bonded together at one end to form a thermocouple junction. The opposite ends of the two sensor wires are electrically connected to an end of one of the at least two sensor electrodes. In addition, at least one heater wire is connected between the at least two heater wire electrodes. The center point of the heater wire may be physically but not electrically connected to the thermocouple junction using a ceramic material.
The sensor electrodes, heater electrodes, sensor wires and heater wires are enclosed in a housing which is filled with a gas. In a preferred embodiment the housing is filled with ambient air, however, additional gases such as hydrogen, helium, or argon may also be used.
When a current is applied to the heater wire, the heater wire becomes hot and warms the gases surrounding it. The gases surrounding the heater wire expand with the increased heat. The heated gas becomes less dense and is quickly replaced with cooler, more dense air. The resulting displacement forces the warm air to rise above the cooler, denser air resulting in convection flow. The thermocouple formed from two dissimilar metals, produces a DC voltage that is proportional to the difference in temperature between the thermocouple Hot Junction and the Cold Junction located at the ends of the two thermocouple wires where they are attached to the terminal posts. In addition, a DC voltage can be generated when the Hot and Cold Junctions are at the same temperature and there is a temperature gradient between the Hot and Cold Junctions. The terminal posts are thereafter attached to electrical lead wires. Depending upon the degree of tilt, the thermocouple junction will receive more or less heated air and produce an output voltage accordingly. This output voltage, when applied to the appropriate circuit, can be translated into a corresponding degree of tilt.